We explore theoretically the feasibility of functionalizing carbon nanostructures for hydrogen storage, focusing on the coating of ${\mathrm{C}}_{60}$ fullerenes with light alkaline-earth metals. Our first-principles density functional theory studies show that both Ca and Sr can bind strongly to the ${\mathrm{C}}_{60}$ surface, and highly prefer monolayer coating, thereby explaining existing experimental observations. The strong binding is attributed to an intriguing charge transfer mechanism involving the empty $d$ levels of the metal elements. The charge redistribution, in turn, gives rise to electric fields surrounding the coated fullerenes, which can now function as ideal molecular hydrogen attractors. With a hydrogen uptake of $>8.4\mathrm{wt}\%$ on ${\mathrm{Ca}}_{32}{\mathrm{C}}_{60}$, Ca is superior to all the recently suggested metal coating elements.

• Received 6 December 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.206806